Actually, they are genetic. Our cells have receptors which are triggered by exposure to pathogens; that's how we "learn" to fight off a disease. And not everyone has the same receptors, which ones you posses is a genetic trait. It's one of the reasons that Native Americans were so vulnerable to European diseases, they hadn't evolved the receptors that would have enabled them to build up defences against the diseases the Europeans had lived with for generations. If your hypothesis was correct we wouldn't see different populations having different reactions to the same disease.
Label: Sarcastic